1. Field of the Invention
The present invention generally relates to battery protection devices for use in engine powered vehicles and equipment which rely upon a battery to start the engine and, more particularly, to protective circuitry responsive to variation in the electrical characteristics of a battery due to different drain rates.
2. Description of the Prior Art
The battery used to start the engine in engine powered vehicles and equipment may be drained by the operation of associated electrical apparatus or by short circuits. The problem of preserving sufficient charge in the battery so that it is usable to start the engine has been variously resolved in the art. Conventional practice is to recharge the battery when the engine is running, so problem resolution focusses on drains which occur when the engine is not running. Common to such resolutions is the objective of detecting a battery discharge condition beyond which the battery may not be able to start the engine, and the objective of then interrupting further drain upon the battery, except when the engine is running.
U.S. Pat. No. 5,381,295 to the present applicants (Rund, et al.) discloses relevant prior art and those disclosures are incorporated herein by this reference.
It is common in the prior art to rely upon battery voltage to indicate the state of charge in the battery. In Rund et al., for example, there is described a cutoff voltage which, in relation to a reference voltage, indicates battery drain and triggers operation of the protective device, which then serves to disconnect the battery while there is still enough energy to start the vehicle engine. Similar use of a single low voltage point indicative of a battery drain condition is disclosed in a variety of other patents, including U.S. Pat. No. 4,493,001 to Sheldrake, U.S. Pat. No. 3,522,481 to Terzuc, U.S. Pat. No. 3,462,647 to Russell, U.S. Pat. No. 3,474,296 to Rickey, U.S. Pat. No. 3,623,131 to Russell, U.S. Pat. No. 3,646,354 to Von Brimer, U.S. Pat. No. 3,656,045 to Frezzolini et al., U.S. Pat. No. 3,648,145 to Meyer et al., U.S. Pat. No. 4,412,267 to Hansen, U.S. Pat. No. 4,137,557 to Ciarniello et al., U.S. Pat. No. 4,005,344 to Gaind et al., and U.S. Pat. No. 4,313,079 to Lee.
However, a cutoff voltage suitable for battery drain conditions of greatest concern--such as leaving the headlights on--will not be the same as a cutoff voltage suitable for lesser drain conditions, such as leaving the dome light on or leaving a vehicle parked for an extended period of time where there are electronic devices such as clocks or even the protective circuitry itself which are designed to draw a small amount of battery power continuously.
As is well known in the battery art, storage batteries operate by means of a chemical process, wherein electrons flow from the positive plate of each battery cell to the negative plate through an electrolyte. For example, in a lead-acid storage battery the positive plate is lead dioxide, the negative plate is lead and the electrolyte is sulfuric acid. The voltage across each cell is about 2.1 volts and is a function of the concentration of the electrolyte and the differences between the metals in the positive and negative plates. When the battery is connected to an external load, lead from lead dioxide in the positive plate combines with the sulfate radical in the sulfuric acid electrolyte to form lead sulfate, and oxygen from the lead dioxide combines with hydrogen from the sulfuric acid to form water. At the same time, a similar process converts lead in the positive plate to lead sulfate. As these processes continue, the positive and negative plates are both changing to lead sulfate, and therefore becoming more alike, and the water which is formed is diluting the sulfuric acid. These changes reduce the state of charge of the battery, and also reduce the voltage.
However, the rate at which electrons are absorbed at the negative plate is limited by the surface area of the plate. At high current drains electrons accumulate at the negative plate without being absorbed, thereby reducing battery voltage below a level which would accurately reflect the state of charge of the battery. When the high current drain stops (e.g. when the starter motor is shut off), the electrons which have not been absorbed will find their way back to the positive plate, thereby increasing battery voltage back to a level reflecting the state of charge of the battery.
By contrast, at low current drains the electrons flowing through the electrolyte will be absorbed at the negative plate and will not accumulate. Consequently, the voltage of the battery under a low current drain will slowly decrease, and will more accurately reflect the state of charge of the battery.
U.S. Pat. No. 4,493,001 to Sheldrake discloses a circuit and apparatus which is responsive to a prior art defect in voltage based battery protection devices, namely, the false detection of a battery discharge condition due to abrupt voltage drops incident to operator entry of the vehicle. Thus the disclosure in Sheldrake recognizes that low battery voltage by itself does not always indicate when there will not be enough power in the battery to start the vehicle. However, Sheldrake's approach is to compensate for a particular circumstance where the battery voltage is not a valid indicator, namely, when vehicle entry lights are turned on and a low voltage spike is produced. Sheldrake provides a three minute delay, which is enough time to start the engine after entry of the vehicle.
More recently, U.S. Pat. No. 4,902,956 to Sloan discloses means for measuring battery charge more accurately by detecting the battery discharge rate and using that discharge rate to variably set a threshold voltage level for triggering operation of the battery protection circuitry. This technique provides for disconnecting the battery at a lower voltage under heavy load conditions, but at a higher voltage when battery drain is due to parasitic loads over an extended period of time. Sloan recognizes that a voltage trigger suitable for heavy load conditions (such as leaving the headlights on) will not be suitable for light load conditions over a longer period (such as leaving glovebox light on). Sloan resolves this difficulty by detecting the drain rate of the battery. However, this solution adds to the complexity of the circuitry of the protection device, because of the need to detect and use the additional battery discharge rate information. In the preferred embodiment, Sloan provides for a microprocessor implementation.
A more recent Sloan patent, U.S. Pat. No. 5,089,762, further adds to detection circuit complexity by adding temperature as a factor affecting the appropriate voltage for triggering operation of the battery protection circuitry. Again, a microprocessor is used to set the appropriate triggering voltage. A further Sloan patent, U.S. Pat. No. 5,332,958, provides for a microprocessor controlled upward adjustment of the battery cutoff voltage over time so that the battery will be disconnected after vehicle storage or during other long periods of disuse.
What is needed is a simpler approach which relies upon measurements of battery voltage, yet is able to protect the battery under a variety of drain conditions.